c++ - Use of templates in function parameters and return type

0
votes

Question 1 : I'm trying to use a struct with a template in function parameters and return type while declaring function.

template <typename T>
struct my_struct { 
  T value; 
}; 

my_struct<T> func(my_struct<T> input_1, my_struct<T> input_2);

When I tried the above code I'm getting

error: ‘T’ was not declared in this scope

However, when declaring with concrete data_type, it compiles.

my_struct<float> func(my_struct<float> input_1, my_struct<float> input_2);

Does it mean, I've to declare function for all data types that I expect? Clearly, I misunderstood the concept here. Can someone explain this?

Update: The above mentioned issue is resolved by declaring function as template.

Question 2 : Now, when i try to invoke the function, I'm getting the below error. What am i doing wrong here?

template <typename T> struct my_struct { T value; };

template <typename X, typename Y, typename Z>
my_struct<X> func(my_struct<Y> input_1, my_struct<Z> input_2) {
  my_struct<X> out;
  out.value = input_1.value + input_2.value;
  return out;
}

int main(int argc, char const *argv[]) {
  struct my_struct<int> t1 = {1};
  struct my_struct<float> t2 = {2.0};
  struct my_struct<float> out = func(t1, t2);
  return 0;
}

error: no matching function for call to ‘func(my_struct&, my_struct&)’

2
c++

2 Answers

1
votes

You have to declare func as template if you want it to be a template. T is only declared inside the definition of the template my_struct.

template <typename T>
my_struct<T> func(my_struct<T> input_1, my_struct<T> input_2);

Or if you actually want no template, but an instantiation of my_struct, eg for int

my_struct<int> func(my_struct<int> input_1, my_struct<int> input_2);
2
votes

You have to define the function template too:

template <typename T>
my_struct<T> func(my_struct<T> input_1, my_struct<T> input_2)
{
    // ...
}